Substrate Factors Determine Roadside Vegetation Structure and Species Richness: A Case Study Along a Meridional Gradient in Fennoscandia

This study assessed the effects of road-related alteration of substrate, including increased salinity, on vegetation along a meridional gradient in Fennoscandia. Vegetation community composition were surveyed in 29 randomly selected 1-m2 sized roadside plots. Number of plant species and plant cover (%) on the plots were positively interrelated (p < 0.0001). Both variables also decreased towards the north and with increasing coarseness of the substrate. Canonical correspondence analysis (CCA) indicated that roadside vegetation diversity and composition were most related to the importance of the road (i.e. its size and traffic intensity) and substrate pH. Road importance affects plant dispersal, whereas substrate pH was found to be a factor limiting growth. CCA indicated also that vegetation composition was affected by the meridional gradient and by the substrate salinity; both substrate salinity pH and salinity were not related to meridional gradient. Our results indicate that roadside vegetation diversity and composition is driven by natural and anthropogenic factors.

Substrate Factors Determine Roadside Vegetation Structure and Species Richness: A Case Study Along a Meridional Gradient in Fennoscandia

Bull Environ Contam Toxicol
Substrate Factors Determine Roadside Vegetation Structure and Species Richness: A Case Study Along a Meridional Gradient in Fennoscandia
Małgorzata Jaz´wa 0 1 2
Waldemar Heise 0 1 2
Beata Klimek 0 1 2
0 Institute of Environmental Sciences, Faculty of Biology and Earth Sciences, Jagiellonian University , Gronostajowa 7, 30-387 Krako ́ w , Poland
1 Institute of Botany, Faculty of Biology and Earth Sciences, Jagiellonian University , Kopernika 27, 31-501 Krako ́ w , Poland
2 & Beata Klimek
This study assessed the effects of road-related alteration of substrate, including increased salinity, on vegetation along a meridional gradient in Fennoscandia. Vegetation community composition were surveyed in 29 randomly selected 1-m2 sized roadside plots. Number of plant species and plant cover (%) on the plots were positively interrelated (p 0.0001). Both variables also decreased towards the north and with increasing coarseness of the substrate. Canonical correspondence analysis (CCA) indicated that roadside vegetation diversity and composition were most related to the importance of the road (i.e. its size and traffic intensity) and substrate pH. Road importance affects plant dispersal, whereas substrate pH was found to be a factor limiting growth. CCA indicated also that vegetation composition was affected by the meridional gradient and by the substrate salinity; both substrate salinity pH and salinity were not related to meridional gradient. Our results indicate that roadside vegetation diversity and composition is driven by natural and anthropogenic factors.
Ecological corridors; Road ecology deposition; Substrate; Traffic system
-
Alteration of soil properties along roads is assumed to have
a negative impact on vegetation diversity and structure
(Brown and Gorres 2011; Mu¨ llerova´ et al. 2011;
Malinowska et al. 2015)
. The synergistically operating factors
influencing plant growth include high insolation and
extreme temperatures along tarmac roads, high wind speed,
low organic matter content, increased levels of
road-userelated contamination (e.g. salinity) and continuous
anthropopression (mowing and cutting of vegetation)
(Mu¨ llerova´ et al. 2011; Fan et al. 2014)
. The high level of
environmental stress typical for roadsides leads to the
development of specific plant communities.
Roads, being a major human promoter of urbanisation,
allow for plants to be dispersed by transportation and road
building materials. Roads are ecological corridors which
support the expansion of a taxon’s geographical distribution
(Hayasaka et al. 2012)
. There is growing interest in the
effects of roads on the dispersal and expansion of plants,
especially non-native ones
(Hayasaka et al. 2012; Tyler et al.
2015)
. The sources of alien plants include motor vehicle
traffic and the movement of seed banks in road-building
material or in the gravel applied in winter to minimize
skidding. Alien plants may enrich the local flora but may also
cause an invasion, potentially altering ecosystem processes
and functions
(Hayasaka et al. 2012; Watkins et al. 2003)
.
Another source of alien plants, or of plants not typical for the
surrounding habitat, is routine sowing of fast-growing
grassland species on road verges
(Tikka et al. 2000)
.
The increased climate harshness along a meridional
gradient is one of the most important factors determining
vegetation diversity and composition
(Mannion et al. 2014)
.
Temperature, insolation, precipitation, and the seasonal
distribution of these factors determine decreased alpha plant
diversity (number of species) and change beta plant diversity
(species composition) towards north
(Mannion et al. 2014)
.
Among the different physical and chemical properties of
the substrate, pH and salinity seem to be the most
important factors shaping the composition of roadside
vegetation. Alkaline gravel, often used on roads, may
change substrate pH in the near vicinity, especially in
nutrient-poor environments
(Mu¨llerova´ et al. 2011)
. This
can alter the plant species composition, favouring species
that prefer higher substrate pH
(Rose and Hermanutz
2004)
. Roadside vegetation is often exposed to higher
salinity due to the use of de-icing agents, mainly sodium
chloride (NaCl) and calcium magnesium acetate
(CaMg2(CH3COO)6). For road managements with seasonal
climate, road salt is used along fine gravel to improve road
safety in hazardous icy conditions in winter. Salinity is one
of the strongest environmental factors limiting plant
growth and productivity
(Allakhverdiev et al. 2000)
. Plants
that can survive and grow well under high salinity in the
rhizosphere are called halophytes; such plants employ
many defence mechanisms
(Parida and Dasa 2005;
Skorupa et al. 2016)
. Many plant species can tolerate increased
salinity, but at the expense of their vigour. Fan et al. (2014)
showed that de-icing road salts applied in the Sierra
Nevada Mountains caused increased forest mortality in the
vicinity (\10 m) of roads.
Our study examined vegetation on roadsides in the cold
climate of Fennoscandia. Most of traffic-related emissions
have clearly fallen there since the early 1980s
(see data
published by European Environment Agency, e.g. SOER
2015)
. Thus, we anticipated low levels of traffic associated
contamination, which were, however, not further quantified.
Although the human impact on the environment in Northern
Europe is low relative to densely populated regions, the
expected future climatic warming and changes in land use
may soon lead to a rapid transformation of boreal and
subarctic ecosystems, with species-poor boreal ecosystems
being particularly vulnerable to various disturbances
(Rose
and Hermanutz 2004; Bradshaw et al. 2009; We˛grzyn et al.
2013)
. Knowledge of the qualitative characteristics of
roadside vegetation can help clarify the mechanisms of
floristic homogenisation resulting from long-distance
transport of propagules over land. Such data can also serve as
a baseline for tracking temporal changes in vegetation.
Roadsides may foster the formation of similar
vegetation communities at sites remote from each other. In this
study we surveyed roadside vegetation in the northern part
of the Scandinavian Peninsula, and related the
physicochemical characteristics of roadside substrate to the
vegetation composition along a north–south gradient.
Materials and Methods
Study plots were randomly selected along several roads
leading throughout part of the Scandinavian Peninsula
(Finland and Norway). Figure 1 shows the distribution of
the 29 study plots. The roads were classified in three groups
according to size and traffic load: (1) highways and
national roads, (2) local roads (tarmac), and (3) other local
roads, including paved roads. In each study plot we
des2
ignated representative roadside vegetation patches (1 m )
and made releve´s there. Species were identified in the field,
and the coverage of particular species on the study plot was
recorded as percentage of plot area, meaning 100 % as area
total coverage and 0 % as lack of coverage. Some plant
species were identified only to genus level (Peltigera sp.,
Salix sp.). Means and standard deviations were calculated
for number of plant species and plant cover for all studied
plots.
Material was collected from roadsides and classified as
road building material (aggregate, gravel, chippings)
enriched with organic matter, mainly plant detritus. A sample
was taken from the upper substrate layer (to 10 cm depth)
at the centre of the releve´ in each study plot. To compare
substrate characteristics between the roadside and its
vicinity, soil was also collected at three other distances
from the road (5, 10, 150 m). Substrate and soil were
collected to separate plastic bags and transported to the
laboratory. The collected material was sieved (1 cm mesh)
and stored field-moist at 4 C before further analyses.
The density of substrate and soil was determined as the
weight of known volume of dry material. Dry weight (DW)
was determined after drying of the substrate samples at
105 C for 24 h, and organic matter (OM) content was
determined as the loss on ignition at 550 C for 24 h. Water
holding capacity (WHC) was measured by a gravimetric
method after soaking the substrate and soil for 24 h in
netended plastic pipes immersed in water. Substrate and soil
pH was measured in air-dried subsamples (3 g) shaken in
water (1:10 w/v) for 1 h at 200 rpm. Substrate and soil
conductivity (lmhos cm-1) was determined in the water
slurry and converted to conductivity value at 25 C and then
expressed as actual salinity per organic matter unit.
Fine-fraction subsamples of the roadside substrate were
separated for elements concentration analysis and the final
result was recalculated per total sample weight. The total C
and N contents were analysed in fine-ground dry material
with a CHNS analyser (Elementar Analysensysteme
GmbH). The C:N, C:P and C:S ratios were calculated for
each sample. Total element concentrations in each sample
(Ca, K, Mg, Mn, Na, and P) were determined after wet
digestion of 0.5 g DW of substrate in 10 mL of SupraPure
HNO3 and HClO4 (7:1 v/v) (Sigma-Aldrich). Elements
concentrations were measured by atomic absorption
spectrometry (AAS, Perkin Elmer) with a flame (type
AAnalyst200) or graphite furnace (type AAnalyst200) nebuliser,
depending on concentration). Only the P concentration was
measured with a flow-injection analyser (MLE Gmbh
Dresnen, type FIA compact). To control accuracy, four
blank samples and three replicates of standard certified
material (CRM025-050, Sandy Loam 8, RT Corp.) were
analysed with the substrate and soil samples.
One-way ANOVA was performed to test the
significance of differences in means for substrate and soil
physicochemical characteristics between the studied road
distances. Normality of the data distribution within groups
was assessed using the Shapiro–Wilk test and the data were
transformed if necessary. Differences were considered
statistically significant at p \ 0.05. Pairwise differences in
means were tested with Tukey’s test. Simple regressions
were performed to assess the relation between substrate
density (g cm-3 DW) and salinity (expressed as mg g-1
OM) as a proxy of salinity stress to plant roots. We checked
also the relationships between these substrate properties
and latitude.
Simple regression was performed to assess the
relationship between number of plant species and plant cover
(p \ 0.05). Canonical correspondence analysis (CCA) was
used to examine the correlation between vegetation
composition in the 29 studied plots with latitude, road traffic
character, and physicochemical properties of substrate. The
final set of factors giving the best fit of the CCA model to
vegetation data was chosen and checked for relationship
with latitude with simple regressions. Additionally, simple
regressions were performed to relate the number of plant
species and plant cover (%) to the variables used in CCA
analysis and to relate substrate pH and salinity with their
location along meridional gradient.
The ANOVA and regression analyses were done with
Statgraphics Centhurion XVI (StatPoint Technologies Inc.,
Warrenton VA, U.S.A.), and the CCA analyses used PAST
2.17c (Natural History Museum, University of Oslo,
Norway).
Results and Discussion
We identified 64 plant species on the 29 studied plots. The
number of species per plot ranged from 2 to 15 (mean 8.1,
S.D. 3.4). Scots pine (Pinus sylvestris L.) was the most
common species, found on 19 of the plots. Only small pine
seedlings were detected on roadsides (\15 cm of main
shoot length). Twenty-two of the plant species were found
only on single plots (e.g. Equisetum arvense, Plantago
maritima, Orthilia secunda). The roadside vegetation was
composed mainly of species with a wide ecological
amplitude, such as the commonly sown grass Festuca
ovina or dwarf forms of Pinus sylvestris. The presence of
species typical for wetlands (e.g. Parnassia palustris,
Ledum palustre) reflected the fact that some roads were
constructed near bogs or even across them (as levees), or
along roadside ditches. The majority of species occurring
in single stands were common boreal zone species, but the
presence of a Plantago maritima stand is especially
interesting. Plantago maritima is typical for seaside, but this
species was found at a stand 180 km from the coast,
suggesting a large role of roads in the dispersal of this plant.
It is thought that boreal ecosystems are susceptible to
alien plants invasions, especially in anthropogenically
disturbed habitats
(Rose and Hermanutz 2004)
. Increased
soil pH is considered a factor fostering such invasions
(Rose and Hermanutz 2004)
, especially in boreal acid
podsolic soils with low calcium content
(Ranta et al. 2015)
.
However, given the type of area we studied, the absence of
invasive alien plant species in the 29 randomly selected
plots may suggest that northern Fennoscandia has a low
rate of alien species invasion, yet.
The mean plant cover in the releve´s ranged from 5 % to
84 % (mean 43.0 %, S.D. 25.1 %). Festuca ovina gg.,
Empetrum nigrum and Betula pendula had the highest
cover on all plots if taken together. Also common were
Trifolium pratense and Trifolium repens, species known to
be resistant to physical damage, which are a frequent
component of seed mixtures sown on road verges
(Tikka
et al. 2000)
. There was a highly significant positive
relationship between number of plant species and plant cover
(p \ 0.0001; r = 0.70) (Fig. 2).
Tilman et al. (1997)
observed a similar relationship in their classical study of
grasslands; however, they related plant diversity to the
plant biomass, which is not directly linked to plant cover.
Substrate density was highest in samples from roadsides,
indicating a higher fraction of inorganic matter than in
samples collected further from the road (Table 1). The
organic matter content of roadside substrate was very low,
as was its water holding capacity (Table 1). Substrate pH
and salinity were higher on roadsides that at sites further
from the road (Table 1). Higher substrate pH on roadsides,
also found by other authors, may result from the use of
alkaline gravel in road construction
(Cˇ ernohla´vkova´ et al.
2008)
. Simple regressions indicated a strong positive
correlation between substrate density (g cm-3) and salinity
(g kg-1 OM) (p \ 0.0001; r = 0.64), confirming that
salinity is linked to the composition of road-building
material and road maintenance practices. Both substrate pH
and salinity on roadsides were not related to meridional
gradient (p = 0.5268 and p = 0.7895, respectively).
Salinity is a strong stressor for roadside vegetation as well
as for soil organisms, including microorganisms beneficial
to plants
(Cˇ ernohla´vkova´ et al. 2008)
. However, substrate
density was related to meridional gradient and the higher
latitude, the higher substrate density (coarser substrate
structure) was found (p = 0.0295). The coarser substrate
under northward stands is attributable to lower organic
matter content (less plant detritus) on roadsides. Lower
plant growth and plant size may be due to the colder
climate accompanying the difference in habitat.
Elements content and their ratios were lowest in
roadside substrate (Table 2), showing that roadsides furnish an
extremely poor environment for vegetation. The coarse
structure of roadside substrate exposes it to washout and
consequent depletion of biogens such as nitrogen and
sulphur.
In turn, the concentrations of other essential elements
(K, Na, Ca, Mg) were significantly higher in roadside
substrate than in samples collected further from the road
(Table 3). This result reflects the higher mineral fraction on
roadsides than in samples collected further from the road
(Cˇ ernohla´vkova´ et al. 2008)
. The ‘road effect’ on substrate
characteristics and element content disappeared between 5
and 10 m from the road; the majority of measured
physicochemical characteristics did not differ between
samples collected 10 m from the road and those collected
150 m from the road.
The final set of environmental variables data in CCA
analysis included latitude, road traffic character, substrate
pH and its salinity. None of physicochemical properties of
substrate used in CCA was related to latitude as showed
with simple regressions (p [ 0.05). The first two CCA axes
calculated for vegetation diversity and composition
explained 41.9 % (p = 0.0495) and 33.8 % (p = 0.0198)
of the variance respectively (trace p = 0.0099). The first
CCA axis was strongly and positively related to pH (0.82)
and slightly less to salinity (0.22), and negatively related to
latitude (-0.26) and road category (-0.11) (Fig. 3). Road
category had the highest relevancy on the second axis
(0.77). The second CCA axis was also positively related to
salinity (0.28), and negatively related to latitude (-0.40)
and substrate pH (-0.01) (Fig. 3).
Road size and traffic intensity affected the roadside
vegetation. Some plant species were found only along
small local roads (Rumex acetosella, Trifolium
ochroleucum, Poa pratensis; Fig. 3). This may be related to lower
levels of physical damage to plants along small local roads,
or to differences in road verge management there, as
compared to highways. Such effect was reported by
Forman and Alexander (1998)
in their review on major
ecological effects caused by roads.
Roadside vegetation diversity and composition were
strongly related to substrate pH even though the variability
of roadside substrate pH was low (Table 1). Such
Asterisked variables differ significantly among distances (p \ 0.05). Different small letters in superscripts indicate statistically significant
differences between spots assessed at different distances from the road
N-NO3-*
(mg N kg-1 DW)
26.2a (±50.5)
59.8b (±78.7)
72.6b (±73.1)
70.1b (±59.1)
0
5
10
150
Asterisked variables differ significantly between distances (p \ 0.05). Different small letters in superscripts indicate statistically significant
differences between spots assessed at different distances from the road
phenomenon was also found by
Rola et al. (2015)
. In
contrast, the effect of salinity on vegetation was relatively
weak despite the high variability of salinity (Table 1;
Fig. 3). Enhanced salinity level affects soil structure,
fractions dispersion, permeability and osmotic potential,
and leads to loss of soil stability and also to osmotic stress
on vegetation. Salinity stress may affect plants through
reduced soil microorganisms viability
(Cˇ ernohla´vkova´
et al. 2008)
. Moreover, high salinity from de-icing agents
may mobilise heavy metals in the roadside environment, as
shown in a Swedish study
(Ba¨ckstro¨ m et al. 2004)
, and
metals can be a significant factor limiting the emergence of
plants cover along road (Bae et al. 2015). In our study,
white clover Trifolium repens and red clover Trifolium
pratense were found to be more tolerant than other plant
species to increased salinity. These plants are generally
thought to be resistant to environmental stress
(Stoychev
et al. 2013)
. Achillea millefolium, Hieracium sp.,
Taraxacum officinalis and some Polytrichum mosses were also
found to be more resistant to substrate salinity. On the
other hand, the location of these species on the CCA plot
may also suggest a preference for warmer end of studied
meridional gradient, as the effect of salinity on roadside
vegetation diversity and structure was ran counter to
latitude and substrate density (Fig. 3).
Roadside vegetation diversity and composition were
related to latitude; there was north–south gradient of
species occurrence (Fig. 3). Decreasing mean annual
temperature between the outmost stands towards the north (from
ca 2.5 to 0.5 C) may affect vegetation diversity and
structure through exclusion of species with divergent or
narrower ranges of ecological tolerance. This has been
observed commonly for different groups of organisms in
many ecosystems
(Mannion et al. 2014)
.
Simple regressions relating number of plant species
per plot and percentage of plot cover to the variables
used in CCA analysis showed that only latitude affected
those two parameters, and these relations were negative
(r values -0.46 and -0.56, p values 0.0122 and 0.0015,
respectively). In other words, regression analysis showed
that plant species number decreased with the increase of
climate harshness toward the north. Road traffic level
and substrate pH and salinity, which both were not
related to meridional gradient, had no direct effect on
plant species number or plot cover, but these factors
affected plant community composition (beta diversity). In
turn, substrate density (coarser substrate structure) was
positively related to latitude and these suggest that
substrate density also affect negatively on plant species
number and plot cover.
Summing up, here we showed that latitude (climate) and
substrate density are factors determining plant alpha
diversity on roadsides, whereas local conditions—traffic
level, substrate pH and salinity—determine the community
composition of roadside vegetation, that is, its beta
diversity. Roadside vegetation in boreal regions is subjected to
various disturbances, which affect them through various
ways. We showed that substrate pH and traffic load are the
most important factors shaping roadside vegetation in
northern Fennoscandia.
Acknowledgments Michael Jacobs line-edited the paper for
submission. The research leading to these results has received funding
from the European Union Seventh Framework Programme [FP7/
2007-2013] under Grant Agreement No. 262693 [INTERACT] and by
Jagiellonian University (subsidy WBNoZ/INoS/DS759).
Open Access This article is distributed under the terms of the
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tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
Fes.ovi.2—Festuca ovina agg., Fes.rub.—Festuca rubra, Fes.tra.—
Festuca trachyphylla, Ger.syl.—Geranium sylvaticum, Hie.mur.—
Hieracium murorum, Hie.sp—Hieracium sp., Led.pal.—Ledum
palustre, Luz.mul.—Luzula multiflora, Luz.sp—Luzula sp.,
Mel.syl.—Melampyrum sylvaticum, Ort.sec.—Orthilia secunda,
Par.pal.—Parnassia palustris, Pel.leu.—Peltigera leucophlebia,
Pic.alb.—Picea abies, Pin.syl.—Pinus sylvestris, Pla.maj.—Plantago
major, Pla.mar.—Plantago maritima, Poa ann.—Poa annua,
Poapra.—Poa pratensis, Poasp—Poa sp., Ran.ace.—Ranunculus acris,
Rhi.min.—Rhinanthus minor, Rum.ace.—Rumex acetosella,
Sal.cap.—Salix caprea, Sal.lap.—Salix lapponum, Sal.pen.—Salix
pentandra, Sal.sp—Salix sp., Sal.sta.—Salix starkeana, Sol.vig. –
Solidago virgaurea, Tan.vul.—Tanacetum vulgare,
Tar.off.—Taraxacum officinalis, Tri.och.—Trifolium ochroleucum,
Tri.pra.—Trifolium pratense, Tri.rep.—Trifolium repens, Tri.sp—Trifolium sp.,
Vac.myr. –Vaccinium myrtillus, Vac.vit.—Vaccinium vitis–idaea
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